A large number of liquid crystal displays (LCDs) utilizing optical (refractive index) anisotropy (Δn) (hereinafter sometimes simply referred to as Δn) and dielectric anisotropy (Δε) (hereinafter sometimes simply referred to as Δε) characteristic of a liquid crystal compound have been produced. LCDs have been widely applied to watches, calculators, various measuring instruments, automotive panels, word processors, electronic notebooks, mobile phones, printers, computers, TV sets, etc. with demand increasing year by year. A liquid crystal compound exhibits an inherent liquid crystal phase between a solid phase and a liquid phase. The liquid crystal phase is roughly classified into a nematic phase, a smectic phase, and a cholesteric phase. For display applications, a nematic phase is most widely used. Number of display modes have been proposed for LCD application, including dynamic scatter (DS), guest host (GH), twist nematic (TN), super twist nematic (STN), thin film transistor (TFT), and ferroelectric liquid crystal (FLC). Drive systems known for LCD application include static drive, time division drive, active matrix drive, and dual frequency drive.
It is known that the threshold voltage of an electric field effect type LCD using a liquid crystal composition having a positive dielectric anisotropy Δε is in general inversely proportional to the square root of the Δε. In recent years, a liquid crystal material with a decreased threshold value has been demanded particularly for application to twist nematic (TN) mode LCDs that have now mostly come to adopt a battery drive system. To meet the demand, a liquid crystal material with a large positive Δε is of importance.
Having a large Δε, nitrile compounds including a 4-(p-alkylcyclohexyl)benzonitrile have been employed as a liquid crystal material for TN mode LCDs or super twist nematic (STN) mode LCDs. However, because these nitrile compounds are liable to entrap ionic impurities, they are inapplicable to active matrix drives requiring high resistivities (1012 Ωcm or higher). Therefore, a liquid crystal material with high resistivity and large Δε has been awaited.
The viscosity of a liquid crystal composition influences the response time of LCDs. The lower the viscosity, the shorter the response time. Accordingly, it is desirable for the components compounded into a liquid crystal compositions to have low viscosities.
The refractive index anisotropy exerts large influence on visual characteristics of LCDs. The contrast increases with an increase of refractive index anisotropy, and the viewing angle widens with a decrease of the anisotropy. In recent years there is a trend toward liquid crystal materials with a small Δn, namely a wide viewing angle.
An NI point governs the temperature range in which a liquid crystal material shows a liquid crystal state. A liquid crystal material having a higher NI point exhibits a liquid crystal state at a higher temperature.
Compounds terminated with a fluoroalkyl(oxy) group exhibit positive dielectric anisotropy and hardly entrap ionic impurities. They are known as liquid crystal materials capable of developing characteristics required particularly of active matrix drive systems, such as high resistivity, high voltage holding ratio (VHR), and a low ion density. Many compounds having a fluoroalkyl(oxy) group introduced therein have hitherto been proposed. For example, JP-A-55-72143, JP-A-55-40660, JP-A-61197563, JP-A-56-12322, JP-A-58-154532, JP-A-58-177939, JP-A-58-210045, JP-A-5978129, and JP-T-6-500343 propose various kinds of fluoroalkyl-containing compounds. JP-T-1-503145 discloses an electro-optical display device using a compound containing a fluoroalkyl group. JP-T-3-502942 proposes an active matrix LCD using a compound having a fluoroalkyl(oxy) group.
Nevertheless, the fluoroalkyl-containing compounds specifically described in these publications are still unsatisfactory in terms of the demand for low viscosity and a broader temperature range for a nematic phase.